How to stop rendering a renderlayer at any moment of the animation?
Renderlayers have a checkbox next to them which controls whether or not the renderlayer is rendered. 

This checkbox can be animated (like most options in blender) by pressing I while hovering over the check box or by pressing  RMB> Insert keyframe to insert a keyframe.

Note that this will make the renderlayer node not output an image, so your compositing setup must be carefully designed to account for that.

If the layer you want to toggle has no alpha transparency, a simple alpha over node will let you put another image in place of the renderlayer when it is not enabled (in this example, I used a mask node to provide a solid color the same size as the render):



If the layer does have alpha transparency, this won't work.

Instead you'll have to use an animated mix setup or something.